Oxidation resistant cellulosic pellicle and process for the manufacture thereof



United States Patent Office 3,091,554 Patented May 28, 1963 3,091,554OXIDATION RESISTANT CELLULOSIC PELLICLE AND PROCESS FOR THE MANUFACTURETHEREOF Charles M. Rosser, Wallingford, and Richard A. Glinski, CliftonHeights, Pa., assignors to American Viscose Corporation, Philadelphia,Pa., a corporation of Delaware No Drawing. Filed July 1, 1960, Ser. No.40,149

8 Claims.

This invention relates to improved oxidation resistant cellulosicsheets, methods for preparing said sheets, and storage batteriesincorporating said sheets as separators.

Various processes and articles of manufacture wherein cellulosic sheetsor films are used require that they have improved resistance to oxygenand oxides. A prime example of a use of a cellulosic sheet or filmrequiring improved oxidation resistance is in storage batteriesparticularly of the zinc-silver electrode type employing one or morelayers of semi-permeable membranes such as regenerated cellulose as aseparator between the electrodes and the alkaline electrolyte. Theperiod of effective operation of this type of battery is limited by thedeterioration of the separator caused by oxidation due in part toprolonged contact with the alkaline electrolyte and withelectrolytically formed silver peroxide. Several methods have beenemployed to increase the life of the cellulosic membrane therebypermitting an increase in the number of charging cycles. These includeincreasing the number of layers of the film and/ or incorporating aprotective coating on the surface of the film, and/or incorporatingvarious antioxidant materials in the coat ing or in the semipermeablemembrane.

Another use for cellulosic membranes having improved oxidationresistance is in dialyzing processes incorporating cellulosicsemi-permeable membranes for the separation of various components in asolution. The need for improved oxidation resistant, semi-permeablemembranes in dialyzing processes employing strong oxidizing material ormaterial promoting oxidation is obvious.

It is an object of the present invention to provide an improvedoxidation resistant, cellulosic pellicle.

It is another object of this invention to provide a process for theimprovement of the oxidation resistance of cellulosic sheets and films.

It is still anotherobject of this invention to provide a methodemploying a series of steps each of which inv combination unexpectedlyimproves the oxidation resistance of cellulosic sheets and films.

It is a further object of this invention to provide storage batterieshaving a longer useful life.

It is a still further object of this invention to provide an improvedstorage battery employing a zinc-containing electrode, asilver-containing electrode, an alkaline electrolyte solution and ahighly oxidation resistant cellulosic membrane separating the electrodesfrom the electrolyte solution.

In accordance with the present invention an improved oxidation resistantsheet comprises an alkali metal borohydride-treated, non-fibrous,semi-permeable cellulosic pellicle such as regenerated cellulose andhydroxyalkyl cellulose ethers preferably having alkyl groups with from 2to 4 carbon atoms. The borohydride-treated cellulosic pellicle isfurther improved by the incorporation therein of a colloidal metalselected from the group consisting of copper, silver and gold, or by theincorporation of an amine-type antioxidant within the pellicle.Unexpected improvement in the oxidation resistance of theborohydride-treated cellulosic pellicle is experienced by impregnatingit with both the colloidal metal and amine antioxidant.

The non-fibrous, semi-permeable cellulosic pellicles of this inventioninclude homogeneous, continuous single sheets; composite sheets whereinthe non-fibrous cellulosic pellicle is laminated to another or othersheet material; fibrous sheets, for example paper and pressed syntheticresin powders and fibers, impregnated and coated with regeneratedcellulose or hydroxyalkyl cellulose to form a continuous, non fibroussurface; and non-fibrous, porous sheet material of synthetic resinmaterial having cellulose impregnated pores, for example, porouspolyvinyl chloride sheet material impregnated with regenerated celluloseas disclosed in U.S.P. 2,729,694.

The invention also includes the method of improving the oxidationresistance of cellulosic sheets which comprises treating said sheetswith an alkali metal borohydride solution and impregnating with eitherthe colloidal metal or an amine antioxidant compound or both. Colloidalsilver, gold or copper is advantageously applied to the cellulosic sheetby first treating the sheet with a reducible silver, gold or coppercompound and then further treating the sheet with an alkali metalborohydn'de to reduce the metal compound. For example, a regeneratedcellulose pellicle is first steeped in a dilute aqueous solution ofsilver nitrate and then immersed in a dilute aqueous solution of sodiumborohydride to precipitate colloidal silver in situ Within the pellicle.Or, the cellulose pellicle is first steeped in a dilute solution of goldchloride or copper sulfate and then immersed in the borohydride solutionto precipitate colloidal gold or copper within the pellicle. This methodpermits the reduction of the metal compound and the borohydridetreatment of the cellulose pellicle in one step. Cellulosic sheets andpellicle may also be impregnated with colloidal silver, gold or copperin any known manner. A conventional mirroring process employing anammoniacal solution of silver nitrate has been used for the depositionof colloidal silver. This solution is prepared by adding just enoughadditional ammonium hydroxide to nearly completely redissolve freshlyprecipitated silver hydroxide. The cellulose pellicle is steeped in thissolution and then exposed to a reducing solution or atmosphere such asformaldehyde or its vapors. Colloidal gold is similarly deposited by thetreatment of the pellicle withdilute gold salt or oxide solution whichis reduced in situ by a reducing agentsuch as formaldehyde, hydrogen,carbon monoxide, hydrazine or hydroxylamine. The cellulosic sheet orpellicle is treated with the borohydride solution either before or afterimpregnating with colloidal metal.

The storage battery of this invention comprises electrodes, electrolyteand an electrolyte-permeable cellulosic membrane treated with an alkalimetal borohydride, said membrane separating said electrodes and saidelectrolyte, and a battery casing therefor. The invention particularlyconcerns storage batteries having zinc-containing electrodes,silver-containing electrodes and alkaline electrolyte solutions. Theelectrolyte-permeable cellulosic membrane is treated in accordance withthe method of this invention to produce superior oxidation resistantseparators thereby greatly increasing the life of the battery.

The cellulosic sheets of the method of this invention include flexiblecellulosic films and composite sheets having cellulosic surfacecomponents. Examples of cellulosic material include regeneratedcellulose produced by denitration of cellulose nitrate, by the viscoseprocess, by cuprammonium process, by deacetylation of cellulose acetate,etc, and films of hydroxyalkyl cellulose ethers and modificationsthereof. It is preferred, particularly for use as battery separators,that the cellulosic sheets consist of semi-permeable, regeneratedcellulose produced by denit-ration of cellulose nitrate since thesesheets alone have demonstrated somewhat better resistance to alkalineoxidation.

The alkali metal borohydrides in accordance with this invention includesodium, potassium and lithium borohydrides which are applied to thecellulosic sheets in aqueous solutions at concentrations sufiicient totreat the cellulose at a weight ratio of about 1 part borohydride tofrom 2 to 50 parts of cellulose. Aqueous solutions containing from about.02 to about 1% alkali borohydride are usually sufficient for thispurpose. Treatment time for the borohydride application to thecellulosic pellicle ranges from about 1 hour to 5 hours at room temperature.

It is realized that alkali metal borohydrides are known to have beenused in the reduction of cellulose, particularly cellulose pulps.However, the improvement in oxidation resistance of cellulosic filmsafforded by the treatment thereof with the borohydrides as compared tocellulosic films employing other methods of reducing oxidation isentirely unexpected. In addition, the use of the borohydride treatmentpermits unexpected improvement in oxidation resistance when certainother known antioxidant treatments are used in combination therewith. Itappears that the carbonyl groups in cellulosic sheets are reduced by theborohydride and that this reduction leads to stabilization of sensitivelinkages in the cellulose molecule. There is no explanation, however,why this treatment in combination with certain other antioxidanttreatments provides synergistic oxidation resistance.

The amine type antioxidant of this invention is preferably a C -C alkylsubstituted diphenylamine but includes for example, naphthylamines,phenylene diamines, substituted derivatives of these amines and mixturesthereof. The amine antioxidant treatment of the cellulose pelliclegenerally involves the replacement of the water content of the cellulosewith an organic solvent and then treatment of the film with the amineantioxidant in an organic solvent solution. Usually, the cellulosicpellicle is steeped in acetone which replaces the water in thecellulose. Then, the acetone impregnated pellicle is steeped in anorganic solvent such as toluene which replaces the acetone. Thereafterthe pellicle is treated with an amineantioxidant such as octylateddiphenylamine in toluene at the desired concentration. Solutionscontaining from 0.5% and up to the limit of solubility of theantioxidant in the organic solvent are useful and a concentration ofabout 2% by weight is preferred.

In order to test the oxidation resistance of the treated cellulosicsheets of this invention under strong alkaline conditions, which wouldduplicate as far as possible the conditions met by the cellulosicmembrane used as a zincsilver battery separator, a method for testingwas used. This test consists of attaching a weight of 225 p.s.i. to atest film strip and suspending it in a flask containing a strongpotassium hydroxide solution with or without the addition of silverperoxide so that one-half of the film remains above the liquid levelwhile the remaining portion including the weight is submerged in thecaustic solution at elevated temperatures. The silver peroxide may beadded to the solution in order to further accelerate the test. The fiaskcontaining the caustic solution is placed 4 in an oven at 135 F. and thelength of time required for the film to break is recorded.

Test films of regenerated cellulose prepared by the denitration ofcellulose nitrate and having a thickness of about 2 mils were preparedas follows.

Test film A was first washed to remove glycerine and then steeped inacetone. The acetone impregnated film was placed in toluene for 3-0minutes and then placed in a toluene solution of octyl substituteddiphenylamine at a concentration of about 2% by weight for aboutminutes. The film was then dried.

Test film B was first washed to remove glycerine and then steeped in anammoniacal solution of silver nitrate (4 gm./250 ml.). This film wasthen exposed to formaldehyde vapors for 10 minutes to precipitatecolloidal silver therein. The colloidal silver impregnated film was thendried.

Test film C was first washed to remove glycerine and then steeped in anaqueous solution of sodium borohydride (2 gm./250 ml.) for 3 hours atroom temperature. This solution gave aratio of 1 part borohydride to 2parts cellulose. The film was removed from the borohydride solution andwater washed. The washed film was rewashed with a 1% acetic acidsolution, Washed again with water and dried.

Test film D was prepared as was test film C except that a lessconcentrated sodium borohydride solution was used. The solutionconcentration was 0.4 gm./250 ml. of water which gave a ratio of 1 partborohydride to 10 parts cellulose.

Test film E was prepared as were test films C and D except that a lessconcentrated sodium borohydride solution was used. The solutionconcentration was 0.08 gm./ 250 ml. of water which gave a ratio of 1part borohydride to 50 parts of cellulose.

Test film F was first treated as test film D and then further treated astest film A.

Test film G was first Washed to remove glycerine and then steeped in aplain aqueous solution of silver nitrate at a concentration of 4 gm./250ml. of water. When the film was completely impregnated with silvernitrate solution it was placed in an aqueous solution of sodiumborohydride having a concentration of 0.4 gm/250 m1. of water for 2hours at room temperature. Thereafter, the film was Water washed,rewashed in 1% acetic acid solution, washed again with water and dried.

Test film H was prepared as was film G and then further treated as wastest film A.

' The control test film was untreated having the glycer- 1ne removed bywater washing.

The test films were subjected to the previously described oxidationresistance test wherein a 45% solution of KOH was used. The results ofthis test are set forth in the following table.

Table 1 Test Film Hours in 45% KOH at F.

Control (Amin A B O. D. E. (NaBHi) B F. (NaBH4+ Amin 2 G. (ColloidalSilver NaBH H. (oilloidal Silver NaBHi Amine) 288, 363, 503

1 Borohydride treatment with a ratio of 1 part borohydride to 2 partscellulose. ceaBlorohydrlde treatment with a ratio of 1 part borohydrideto 10 parts u ose.

Table 11 Test Film Hours in 45% KOH and AgZOZ at 135 F.

is, 25, 22, 23, 25, 22, 2s 20 28 If 1 69, 70 (NaBHi Amine) 1 45(Colloidal Silver N 3BH4) 90 (Colloidal Silver NaBHi Amine) l (ColloidalSilver Amine) 1 Borohydride treatment with a ratio of 1 part borohydrideto 10 parts cellulose.

To demonstrate this invention employing colloidal gold and copper asantioxidant agents the following data are given.

Test film J was first washed to remove glycerine and then steeped in adilute aqueous solution of gold chloride (4 gm./250 ml.) for 2 hours.This film was then exposed to formaldehyde vapors for about 10 minutesto precipitate the gold particles therein. Thereafter, the goldimpregnated sheet is further impregnated with octyl substituteddiphenylamine in the manner set forth for test film A.

Test film K was prepared by first washing the regenerated cellulose filmto remove glycerine plasticizer. It 0 was then steeped in aqueous goldchloride solution (4 gm./250 ml.) for 2 hours. The saturated film wasthen steeped in an aqueous solution of sodium borohydride (0.4 gm./ 250ml.) for 3 hours at room temperature. The film was then water washed,washed with 1% acetic acid, rewashed with water and dried.

Test film L was prepared in the same manner as test film K and thenimpregnated with the amine antioxidant in the manner set forth for testfilm A.

Test film M was prepared in the same manner as test film L except thatthe gold chloride solution was replaced with an aqueous copper sulfatesolution (4 gm./ 250 ml.).

The above test films were subjected to the test procedures described forTables I :and II and the results are set forth in the following table.

Table III The results set forth in the above tables demonstrateconclusively the excellent oxidation resistance of the films and sheetsof this invention and the unexpected improvement provided by thedisclosed process. Known treat ment of films to reduce oxidation arecomparatively in effective when used alone or even when combined as withtest film I. However, the oxidation resistance of films treated with theborohyd-ride is greatly improved while the combined treatments includingthe borohydride produce a synergistic improvement well over the expectedresult.

Colloidal silver impregnation with reduction of the silver salt byformaldehyde or the like appears to be far inferior in producingoxidation resistance than when the borohydride is used to simultaneouslyreduce the silver salt and treat the cellulose film or pellicle.

Impregnation with both gold particles and diphenylamine producesunexpected improvement in oxidation resistance either when the gold saltis reduced with ordinary reducing agents such as formaldehyde or withsodium borohydride. However, the great increase when using theborohydride treatment is totally unexpected even when compared to theexcellent results obtained using formaldehyde as the reducing agent forthe gold salt.

Further improvement in the method of this invention by treatment ofcellulosic pellicles with aqueous alkali metal borohydrid-e solutions inthe presence of a water soluble salt of a metal of the group includinglithium, magnesium, calcium, strontium and barium is expected inaccordance with U.S.P. 2,898,333. Thus, in the presence of these saltssmaller amounts or less concentrated aqueous solutions can be used totreat the cellulosic pellicle to obtain the desired result.

Various changes and modifications may be made in practicing theinvention without departing from the spirit and scope thereof and,therefore, the invention is not to be limited except as defined in theappended claims.

We claim:

1. A method of improving the oxidation resistance of a non-fibrouscellulosic pellicle which comprises impregnating said pellicle with areducible compound of a metal selected from the group consisting ofgold, silver and copper, reducing said compound of a metal toprecipitate a colloidal metal in situ within said pellicle, and treatingsaid pellicle with an alkali metal borohydride solution at a weightratio of about 1 part borohydride to from 2 to 50 parts cellulose.

2. A method of improving the oxidation resistance of a non-fibrousoellulosic pellicle which comprises impregnating said pellicle with aborohydride reducible compound of a metal selected from the groupconsisting of gold, silver and copper, and reducing said compound of ametal to precipitate a colloidal metal in situ within said pellicle bytreating said pellicle with an alkali metal borohydride at a weightratio of about 1 part borohydride to from 2 to 50 parts cellulose.

3. The method of claim 2 wherein the metal compound is silver nitrate.

4. The method of claim 2 wherein the metal compound is gold chloride.

5. The method of claim 2 wherein the cellulosic pellicle is regeneratedcellulose film prepared by denitration of cellulose nitrate.

6. The method of claim 2 wherein the cellulosic pellicle is also treatedwith an amine antioxidant compound.

7. The method of claim 6 wherein the film is treated with an alkylsubstituted diphenylamine dissolved in an organic solvent.

8. An oxidation resistant sheet comprising an alkali metalborohydride-treated, non-fibrous, semi-permeable cellulosic pellicleselected from the group consisting of regenerated cellulose andhydroxyalkyl cellulose ethers, said pellicle impregnated with (l) acolloidal metal selected from the group consisting of gold and silver,and (2) an alkyl substituted diphenylamine.

References Cited in the file of this patent UNITED STATES PATENTS2,475,538 Baird July 5, 1949 2,511,472 Kmecik June 13, 1950 2,520,963Reeves Sept. 5, 1950 2,602,757 Kantrowitz et a1. July 8, 1952 2,696,515Koren et a1. Dec. 7, 1954 2,729,540 Fisher Jan. 3, 1956 2,898,333Jullander Aug. 4, 1959 3,013,099 Mendelsohn Dec. 12, 1961

1. A METHOD OF IMPROVING THE OXIDATION RESISTANCE OF A NON-FIBROUSCELLULOSIC PELLICLE WHICH COMPRISES IMPREGNATING SAID PELLICLE WITH AREDUCIBLE COMPOUND OF A METAL SELECTED FROM THE GROUP CONSISTING OFGOLD, SILVER AND COPPER, REDUCING SAID COMPOUND OF A METAL TOPRECIPITATE A COLLOIDAL METAL IN SITU WITHIN SAID PELLICLE, AND TREATINGSAID PELLICLE WITH AN ALKALI METAL BOROHYDRIDE SOLUTION AT A WEIGHTRATIO OF ABOUT 1 PART BOROHYDRIDE TO FROM 2 TO 50 PARTS CELLULOSE.